The invention relates to lead-in conductors used in hermetically sealed metal packages. Ordinarily, such lead-in conductors involve a glass insulator which holds the lead-in conductor in place and also insulates it from the metal package. A great deal of development technology has been directed toward the development of suitable glass-metal seals for use in such an application. In the so-called compression seal a glass is selected having a relatively low thermal coefficient of expansion (TCE) and is sealed inside an eyelet in a base metal having a relatively high TCE. When such a combination is cooled from the sealing temperature to low temperature, the metal shrinks more than the glass thereby placing the glass under compression. Glass is quite strong under compression so such a combination is stable. The material of the lead-in is selected to have a TCE about equal to that of the glass or slightly lower. Reference is made to U.S. Pat. No. 2,770,923 which is directed to an aluminum to glass internal seal that is under compression after the seal is completed and to a low melting glass that is workable at a temperature below the melting temperature of aluminum. U.S. Pat. No. 3,035,372 is another example of a compression seal teaching. Here a low TCE lead-in is glass sealed to an opening in a high TCE member such as SAE 1010 steel. Such a seal is made at the glass working temperature which is typically on the order of 1000.degree. C. U.S. Pat. No. 4,349,635 is exemplary of an attempt to provide a low melting sealing glass for compression seals. A lead-free glass is mixed with powdered alumina to produce a mixture that can be sealed at 750.degree.-800.degree. C. This is significantly lower than the 1000.degree. C. prior art glasses but it is still too high. For example, it precludes the use of aluminum which melts at about 660.degree. C. The teaching contained in the above three patents is incorporated herein by reference.
The working temperature of the commonly used glasses for forming compression seals is about 1000.degree. C. The glass described in the above-referenced U.S. Pat. No. 2,770,923 has a working temperature of about 560.degree. C. At these working temperatures any coatings on the seal elements will tend to melt and/or interdiffuse with the base metal. Such coatings are needed to provide a metal surface which can be wire bonded or otherwise contacted to complete the electrical interconnection of the housed parts. If the coating is melted or interdiffused with the metal base its bondability can be destroyed. If applied before the seal is fired such a coating must be made unacceptably thick. Therefore, in the fabrication of hermetic metal packages it is common practice to plate the packages after they are assembled. This is costly because the parts must be handled on an individual basis and, if electroplating is employed, all of the metal parts must be electrically contacted individually. It is greatly preferred that the parts be plated prior to assembly.